1. Field of the Invention
This invention relates to an electrolytic process for the formation of a hard, protective film on the surface of a rectifier metal or alloy thereof.
2. Description of the Prior Art
One of the primary problems associated with the extensive use of magnesium and aluminum is the tendency of these metals to corrode especially when exposed to an environment where an alkali metal halide such as sodium chloride is present alone or in combination with water. A well known process for providing a protective coating for aluminum is the process of anodization. This process has been used to a limited extent with magnesium but the coatings obtained are not highly resistant to attack by corrosive agents such as acids or alkalis, particularly aqueous sodium chloride.
A process is disclosed for anodizing, in fluoroboric acid solutions, aluminum and alloys of aluminum, as described in Chemical Abstracts 75:104365n. A solution containing 0.1 to 2.5% fluoroboric acid was utilized to anodize aluminum and aluminum alloys at a temperature of 0.degree. to 30.degree. Centigrade. A film of high breakdown voltage/thickness ratio was obtained on aluminum indicating a film of compact structure similar in nature to a barrier film. Increased anodizing temperature had an effect similar to higher concentration and increased the deposition of the oxide film. The behavior of the coated aluminum alloys was less satisfactory than the pure metal in resisting corrosion under salt fog conditions.
In U.S. Pat. No. 3,834,999 there is disclosed the electrolytic production of glassy layers on metals and alloys in order to improve corrosion resistance of said metals and alloys wherein the layers are applied by the anodization of the metals and their alloys in anodization baths containing anions such as tungstate, phosphate, arsenate, stannate, molybdate, borate, chromate, and dichromate. The minimum voltage disclosed as useful in the electrolytic anodization of metals or their alloys is about 600 volts.
In U.S. Pat. No. 4,184,926, there is disclosed an anticorrosive coating for magnesium and its alloys which is capable of withstanding the action of strong acids and alkalis prepared by first treating the metal or the alloy thereof with an aqueous solution of hydrofluoric acid to form a fluoromagnesium first layer on the metal surface. After rinsing off the hydrofluoric acid, the metal or alloy is immersed in an electrolytic bath containing an aqueous solution of an alkali metal silicate and an electrical potential is applied between the metal as an anode and a cathode, also immersed in the bath, in order to apply a uniform silicate layer over the first layer on the metal.
In U.S. Pat. No. 3,956,080, there is disclosed a process for coating a rectifier metal such as aluminum or magnesium, said coating being resistant to attack by chemical agents. The protective coating is applied by immersing the metal in a bath containing an alkali metal silicate and applying an electrical potential between the metal, utilized as an anode, and a cathode also immersed in the bath. This reference teaches away from the use of such undesirable ions as bromide, fluoride, iodide, chloride, sulphate, nitrate, and cyanide to form the protective coating.